Apex Large String Performance Test.java

/**
 * Method that creates 4 6MB strings and calculates the time it takes
 */
// Keep track of the total usage
Integer totalCpu = 0;
Integer totalHeap= 0;

// Specify the number of strings
Integer numberOfStrings = 4;

// Generate the strings one by one
for(Integer i =0; i<numberOfStrings;i++){

    // Start measuring limites
    Integer heapS = Limits.getHeapSize();
    Integer cpuS  = Limits.getCpuTime();

    // Example of how a string generation method is used directly in a method, like getting the size of the blob
    System.debug('Blob size: ' + Blob.valueOf(
        generateJoinedListString()
    ).size() + ' - Iteration ' + (i+1) + '/' + numberOfStrings);

    // Finish measuring limites
    Integer cpuE  = Limits.getCpuTime();
    Integer heapE = Limits.getHeapSize();

    // Output results
    System.debug('CPU  Time: ' + (cpuE  - cpuS) );
    System.debug('Heap Size: ' + (heapE - heapS));

    // Calculate totals
    totalCpu +=Integer.valueOf(cpuE  - cpuS );
    totalHeap+=Integer.valueOf(heapE - heapS);
}

// Output results
System.debug('TOTAL CPU  Time: ' + totalCpu );
System.debug('TOTAL Heap Size: ' + totalHeap);

System.debug('CURRENT CPU  Time: ' + Limits.getCpuTime());
System.debug('CURRENT Heap Size: ' + Limits.getHeapSize());


/**
 * Method that generates a 6MB string
 */
public static String generateJoinedListString(){
    
    final String testString = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ01234';

    // Create a list to hold all the lines in our CSV
    // I add an empty line else the file size is 5.999.999
    // It's also easier to debug...
    String[] lines = new String[]{''};
    
    for(Integer rowI=0; rowI < 7500; rowI++){
        
        // Create a list that holds all the columns, representing a single line
        String[] line = new String[]{};

        for(Integer colI=0; colI < 25; colI++){

            // Instead of concatenating strings, we add them to a list
            line.add(testString.escapeCsv());
        }
        // Let's add all column values into a list representing a line and join them with a comma
        lines.add(String.join(line, ','));    
    }

    // Directly return all the joined columns (lines) and join each full line with a return
    return String.join(lines, '\n');
}